Hunting life in Martian rocks

作者：揭皈樯 发布时间：2019-02-26 08:09:04

By Peter Ward (Image: NASA/SPL) Mention the words “Mars” and “meteorite” and you’ll probably think of the 1.9-kilogram hunk of Mars rock discovered in the Allan Hills of Antarctica in 1984. You might not recall its fiddly name, ALH 84001, but exactly 10 years ago this meteorite shot to international fame when a team of NASA scientists claimed it contained evidence of ancient Martian life. The team, led by David McKay of the Johnson Space Center in Houston, Texas, announced in the journal Science that ALH 84001 contained numerous “biosignatures”, including chains of small rod-like structures that they interpreted as fossil bacteria (vol 273, p 924). If they were right, they were the first alien life forms to be encountered by humans. A decade on, the vast majority of scientists who have been involved with this most poked, prodded and analysed of all meteorites now believe that ALH 84001 does not contain evidence of Martian life: all the supposed biosignatures can be accounted for by inorganic processes. The disappointment of ALH 84001 has not spelled the end of the Mars meteorite story, however. Far from it. Just as one debate winds down, another Martian meteorite is being quietly touted as possible evidence that life once existed on the Red Planet. This time it is not small rods but tiny holes that are raising hopes – and ire – once again. The meteorite, called Nakhla, fell to Earth on 28 June 1911 at El-Nakhla in Egypt, reportedly killing a dog. The material arrived in dozens of pieces,